After a hydrocarbon well has been drilled and made safe, well logging operation are generally carried out. The well logging operation serves to measure various parameters of the hydrocarbon well geological formation (e.g. resistivity, porosity, etc . . . at different depths) and in the well-bore (e.g. temperature, pressure, fluid type, fluid flowrate, etc. . . . at different depths). Such measurements are performed by a logging tool. Generally, a logging tool comprises at least one sensor (e.g. resistivity sonde, mechanical sonde, gamma ray neutron sonde, accelerometer, pressure sensor, temperature sensor, etc. . . . ) and measures at least one parameter. It may include a plurality of same or different sensors sensitive to one or more parameters.
The dimension of the well-bore all along the bore is an important parameter for the following reasons.
Firstly, after the hydrocarbon well has been drilled, the well-bore is an open borehole filled with the drilling mud. Generally, the well-bore is cased by cementing operations. In order to correctly plan the cementing operations, the drilling engineers need measurement of the well-bore dimension as a qualitative indication of both the condition of the well-bore and the degree to which the drilling mud has maintained the borehole stability.
Secondly, when the borehole is cased (also known as casing) or tubed (also known as tubing), logging tools are moved up and down in the borehole for gathering data about the various parameters. In order to avoid blocking the logging tools in the borehole, the logging operations require a borehole of sufficient diameter. Thus, the logging engineers need measurement of the well-bore dimension as a qualitative indication of the potential deformations, rugosity or corrosion of the borehole wall.
Thirdly, a well-bore dimension measurement combined with others measurements may be used to determine others characteristic values of the well. For example, the well-bore dimension and a fluid speed measurement (for instance by a spinner) enables to calculate the flowrate of the fluid flowing in the well-bore.
In current logging tools, the measurement of the diameter of a tubing or casing is based on electromechanical devices or ultra-sonic device, also known as caliper. The electromechanical caliper transforms the radial opening of a mechanical arm into a translation movement, which is detected by at least one linear variable differential transformer (LVDT), and provides the dimensions of the completion in one or two directions. A plurality of calipers may also be used to provide multi-point measurements of a tubing or casing (also known as a multi-arm or multi-finger caliper).
The ultra-sonic caliper measures the internal diameter of a casing or tubing using high-frequency acoustic signals. A transducer (in transmit mode) emits a high-frequency pulse that is reflected by the casing or tubing wall back to the transducer (in receive mode). The diameter is determined from the time of flight of this echo and the fluid acoustic velocity. The transducer may be rotated to produce a cross section of the borehole size and full-coverage images of the borehole wall.
Both types of caliper coupled to an appropriate acquisition system provide a caliper log that is a representation of the measured diameter of a borehole along its depth. The electromechanical and ultra-sonic calipers involve electronic system that shows reliability issues when used in harsh environment (high temperature or pressure which are common downhole).